Bioinorganic Chemistry MCQ Quiz - Objective Question with Answer for Bioinorganic Chemistry - Download Free PDF

CONCEPT:

Hydrogen Bonding in Metalloenzymes

• Hydrogen bonding plays a critical role in stabilizing enzyme-substrate complexes in metalloenzymes.
• Specific functional groups or residues act as hydrogen bond donors or acceptors depending on the enzyme’s structure and active site environment.
• These moieties can be amino acids (like histidine or tyrosine) or bridging ligands (like μ-hydroxo or aza(dithiolato)) that form hydrogen bonds with the substrate or intermediates.

EXPLANATION:

• Haemoglobin: Tyrosine is involved in H-bonding to stabilize oxygen binding.
• Nickel-superoxide dismutase: Tyrosine residues are essential for hydrogen bonding with the superoxide intermediate.
• [FeFe] hydrogenase: The aza(dithiolato) bridge is involved in hydrogen bonding within the active site for proton transfer.
• Hemerythrin: μ-hydroxo bridge provides a hydrogen-bonding network to the oxygen species.
• From the table, only Option 3 (C) correctly maps these moieties:
  • Haemoglobin – Histidine
  • Nickel-superoxide dismutase – Tyrosine 
  • [FeFe] hydrogenase – Aza(dithiolato) 
  • Hemerythrin – μ-hydroxo 

Therefore, the correct option is Option 3 (C).

CONCEPT:

Isomer Shifts in Mössbauer Spectra

• The isomer shift in Mössbauer spectroscopy is related to the electron density at the nucleus of the atom. The higher the electron density around the nucleus, the higher the isomer shift.
• The isomer shift is caused by the interaction of the nucleus with the electrons in its surrounding environment. This is most commonly influenced by the nature of the ligands bound to the metal center, particularly whether they are electron-donating or electron-withdrawing.
• Electron donor ligands increase the electron density on the central atom, leading to a higher isomer shift, whereas electron acceptor ligands reduce the electron density, resulting in a lower isomer shift.

EXPLANATION:

• a. Superoxide to oxygen: The enzyme Ni-superoxide dismutase (option ii) catalyzes the conversion of superoxide ions (O₂⁻) to molecular oxygen (O₂). This reaction is typical for superoxide dismutase enzymes, where the electron density is high around the tin center in the enzyme.
• b. Hydrolysis of peptide: The enzyme carboxypeptidase (option iii) catalyzes the hydrolysis of peptide bonds. Peptide bonds are broken to yield free amino acids by this enzyme, where the electron density around the metal center is decreased due to electron-withdrawing properties of the peptide bonds.
• c. Hydroxylation of camphor: The enzyme cytochrome P450 (option iv) catalyzes the hydroxylation of camphor. P450 enzymes add a hydroxyl group (-OH) to substrates like camphor, increasing the electron density around the central metal center, leading to higher isomer shifts.
• d. Primary amine to aldehyde: The enzyme amine oxidase (option i) converts primary amines to aldehydes. It oxidizes amines and reduces the electron density around the metal center, leading to a lower isomer shift due to electron-withdrawing nature of the aldehyde group formed.

The correct match is a - ii, b - iii, c - iv, d - i.

CONCEPT:

Nitrogen Fixation by Nitrogenase

N₂ + 8 H⁺ + 8 e^- → 2 NH₃ + H₂

• Nitrogenase is an enzyme that catalyzes the conversion of atmospheric nitrogen (N₂) to ammonia (NH₃) in biological nitrogen fixation.
• The overall balanced reaction catalyzed by nitrogenase is:
• Here, 1 mole of N₂ is reduced to 2 moles of NH₃, and 1 mole of H₂ is also produced as a byproduct.

EXPLANATION:

• From the equation, we see:
  • x = 2 (moles of NH₃ produced)
  • y = 1 (mole of H₂ produced)
  • z = 8 (protons used)
  • w = 8 (electrons used)
• This stoichiometry is crucial to maintain electron and proton balance during nitrogen reduction.

Therefore, the correct values of x, y, z, and w are x = 2, y = 1, z = 8, w = 8 → Option 4

CONCEPT:

Nitrogenase Enzyme Function

• Nitrogenase is an enzyme complex responsible for the biological nitrogen fixation process, i.e., converting atmospheric nitrogen (N₂) into ammonia (NH₃).
• It consists of two main protein components:
  • Fe protein (iron protein)
  • MoFe protein (molybdenum-iron protein)
• In the absence of nitrogen gas (N₂), nitrogenase does not remain inactive.
• Instead, it catalyzes the reduction of protons (H⁺) to molecular hydrogen (H₂), effectively functioning as a hydrogenase.

EXPLANATION:

• When N₂ is not available, nitrogenase uses its reducing power and ATP hydrolysis to reduce protons and release H₂.
• This alternative function is a natural part of its catalytic cycle, demonstrating its hydrogen-evolving capability.

Therefore, the correct answer is hydrogenase.

CONCEPT:

Biosynthesis of N-Methylhistamine from Histidine

• The conversion occurs in two major enzymatic steps, each requiring a specific coenzyme:
• 1. Histidine to Histamine:
  • Catalyzed by the enzyme Histidine Decarboxylase (HDC).
  • This is a decarboxylation reaction requiring Pyridoxal Phosphate (PLP) — the active form of vitamin B₆ — as a coenzyme.
• 2. Histamine to N-Methylhistamine:
  • Catalyzed by the enzyme Histamine N-Methyltransferase (HNMT).
  • The methylation occurs at the terminal nitrogen (tele-N) of the imidazole ring.
  • This reaction utilizes S-Adenosylmethionine (SAM) as the methyl group donor, and generates S-Adenosylhomocysteine (SAH) as a byproduct.

EXPLANATION:

• PLP is required for the decarboxylation of histidine to histamine.
• SAM is required for methylating histamine into N-methylhistamine.
• Thus, both coenzymes are essential for the complete biosynthesis pathway.

Therefore, the correct answer is A and B — Pyridoxal Phosphate (PLP) and S-Adenosylmethionine (SAM)

Concept:-

• Ferredoxins are non-heme iron-sulfur proteins, which are characterized by the presence of polymetallic systems containing sulfide (S^2-) ions. The iron (Fe) has variable oxidation states.
• They are extremely important in many biological electron transfer processes and these are available in all living bodies. Ferredoxins mainly act as electron Transport proteins in biological redox reactions.
• In these Iron-sulphur proteins, both cysteinyl sulfur and inorganic sulfur as S2- are present. The inorganic sulfur is labile as it can be removed as H₂S on acidification.
• In these electron transport processes, the Fe⁺³/Fe⁺² couple works and both the oxidized and reduced forms of Fe remain in a high-spin tetrahedral geometry.
• The iron-sulfur proteins are represented by nFe-mS. where n denotes the number of Fe cations per protein molecule, S denotes labile sulfur and m denotes the number of labile sulfur sites per protein molecule.
• Rubredoxin is a 1Fe-0S protein (1Fe ferredoxins).

Explanation:-

• Rubredoxins are a class of low-molecular-weight iron-containing iron-sulfur proteins. In contrast to iron-sulfur proteins, rubredoxins do not contain inorganic sulfide.
•  Rubredoxins are denoted as a [1Fe-0S] or an

Fe₁S₀ system.

• It is a one-electron transfer agent, with both Fe⁺² and Fe⁺³ having high spin configurations.
• Fe2+ center has a tetrahedral geometry.
• The oxidation state of the central iron atom changes between the +2 and +3. In both oxidation states, the metal remains high spin configuration.

\(\eqalign{ & {\left[ {{\rm{Fe}}{{\left( {{\rm{RS}}} \right)}_{\rm{4}}}} \right]^{{\rm{2 - }}}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\left[ {{\rm{Fe}}{{\left( {{\rm{RS}}} \right)}_{\rm{4}}}} \right]^{\rm{ - }}} \cr & {\rm{ F}}{{\rm{e}}^{{\rm{ + 2}}}}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{ F}}{{\rm{e}}^{{\rm{ + 3}}}} \cr} \)

• In the reduced state the oxidation state of Fe is +2
• In the reduced state, the electronic configuration of Fe⁺² is 3d64s0. The distribution of electrons in the t2 and e orbitals will be e3 t23. 
• Since in both these sets of orbitals the electrons are symmetrically distributed, thus the complex will show Jahn Teller distortion.
• Thus, the Fe2+ center undergoes Jahn‐Teller distortion.

​Conclusion:-

Hence, the correct statements for rubredoxin are A and C only.

Hemoglobin:

• Hemoglobin is found in all vertebrates and some other animals.
• Hemoglobin contains two parts: heme group and globin protein.
• A porphyrin ring containing a Fe atom is known as a 'heme' group. Hemoglobin has a molar mass of about 64500.
• Hemoglobin is found in red blood cells and is responsible for the red color.
• Each hemoglobin contains four subunits each having a globin protein in the form of folded helix or spiral.
• The globin proteins are again of two types: α and β, alpha-globin contains 142 amino acids and beta-globin contains 146 amino acids.
• The heme group is held in a water-resistant protein pocket.
• Hence, hemoglobin is a heme protein. The structure of hemoglobin is: 

Myoglobin:

• Myoglobin is a protein that has only one heme group per molecule.
• Its main function is to store oxygen in the muscles.
• The myoglobin molecule is similar to a single unit of hemoglobin.
• Myoglobin is a five-coordinate high spin Fe(II) complex with four of the coordination sites occupied by Nitrogen atoms of the porphyrin ring.
• The fifth position is occupied by N-atoms of the imidazole of a histidine residue.
• Hence, it is a heme protein. Structure of Myoglobin is:

Hemocyanin:

• From the name, it may suggest that Hemocyanin contains heme and cyanide, but it contains neither.
• The metal atom present in Hemocyanin is Copper and the meaning of Hemocyanin is 'Blue Blood'.
• Hemocyanin is a copper-containing protein that carries oxygen in some invertebrates such as snails, squids, etc.
• The deoxy form of hemocyanin contains Cu in a +1 oxidation state and is colorless.
• The oxy form of hemocyanin contains Cu in +2 form and is bridged to oxygen via an O₂^2--Cu²⁺ LMCT.
• Hence, hemocyanin is a non-heme protein. The deoxy and Oxy form is shown below:

Cytochrome P-450:

• Cytochrome P-450 is a group of cytochrome that are found in plants, animals, and bacterias.
• Cytochrome P-450 is named so because it absorbs light at wavelength 450nm with their CO complexes.
• The active site of cytochrome is similar to the heme part as found in Hemoglobin and myoglobin except that:
  • The Fe is present in low spin octahedral Fe(II).
  • One S atom of cysteine is coordinated to Fe rather than the N atom of Histidine.
  • The sixth coordination site is occupied by water. Hence, Cytochrome is also a heme protein.

Hence, the non-heme protein is hemocyanin.

Concept:-

The structure of the oxymyoglobin complex is shown below:

Iron is in Fe⁺³ oxidation state, has \(t_2g^{5}\:eg^{0}\) configuration and one unpaired electron (n=1).

Explanation:-

[Fe(ox)3]3−

Fe³⁺(\(t_{2g}^{3}\:e_g^{2}\))=d⁵ high spin octahedral,

No. of unpaired electrons=5

[Fe(CN)6]3−      

Fe³⁺ \((t_{2g}^{5}\:e_g^{0})\) = d⁵ low spin octahedral 

No. of unpaired electrons=1

[NiCl4]2−     

Ni²⁺ \((e_{g}^{4}\:t_{2g}^{4})\)= d⁸ tetrahedral complex, \(eg^{4}\:t_2g^{4}\)

No. of unpaired electrons=2

[Cu(NH3)4]2+

Cu²⁺=d⁹ low spin, square planner

No. of unpaired electrons=1

• In case of complex(es) [Fe(CN)6]3− and [Cu(NH3)4]2+ has one unpaired electron. 
• That is exactly the same as the number of unpaired electron(s) present in the iron center in oxymyoglobin.

Conclusion:-

Therefore B and D is the correct one.

Concept:

• Hemerythrin is Fe- containing  non-heme based protein which functions as O₂ carrier and transporter. It is present in invertebrates.
• Hemocyanin is also present in invertebrates. It is responsible for blue-colored blood in invertebrates. It is Cu based metalloprotein which transports O₂ through out the body.
• Azurin functions in single-electron transfer between enzymes and is found to have anti-cancer properties. It is also a Cu- containing metalloprotein.
• Rieske protein contains Fe-S bonds . The [2Fe-2S] center is responsible for the electron transfer in bacterias, plants, etc.
• Cytochrome P450 is Heme based enzyme. It is mono oxygenase participating in drug metabolism. 
• Tyrosinase is Cu- containing oxygenases that oxidizes tyrosine to melanin which is responsible for pigmentation of skin.

Explanation:

• A - (ii) - (z) Hemerythrin (Fe-based) as well as hemocyanin(Cu-based) are present in invertebrates and functions as O₂  binder and transporter systems. 
• B ‐ (iii) ‐ (x) Cytochrome P450 and tyrosinase are enzymes. Cytochrome P450 is heme based while later contains Cu.But both belongs to the class of Oxygenases.
• ​C ‐ (i) ‐ (y) 2Fe-2S centers of Rieske protein and Cu of  Azurin protein functions in electron transportation. Cu of undergoes oxidation and reduction between Cu(I) and Cu(II) to perform the single electron transfer. Reiske proteins have different type of units. They also facilitates the single electron transport.

Conclusion:

The given proteins and enzymes can be matched with their biological functions as following: A ‐ ii ‐ Z, B ‐ iii ‐ X, C ‐ i ‐ Y

Concept:-

• Iron-Sulfur Clusters: Proteins like rubredoxin and ferredoxins contain iron-sulfur clusters in their active sites. These clusters play a crucial role in electron transfer reactions.
• Redox Reactions in Metalloproteins:The transition between different oxidation states of iron in these proteins involves redox reactions. In the process, electrons are transferred, leading to changes in the oxidation state of the iron atoms.
• Equivalents in Chemical Reactions: The concept of equivalents is related to the amount of a substance reacting in a chemical reaction. In this context, the number of equivalents of H₂S released corresponds to the number of moles of H₂S produced per mole of iron reacting in the protein.

Explanation:-

• The number of equitant of H2S related from rubredoxin will directly proportional to number of labile Sulphurs present

Structre of ruberdoxin

It has zero labile sulphur hence H2S released will be zero

Structure of 2-iron ferredoxin.

It has two labile sulphur hence H2S released will be two

Structure of 4-iron ferredoxin.

It has 4 labile sulphur hence H₂S released will be 4.
Conclusion:-

So, The number of equivalents of H2S gas released from the active site of rubredoxin, 2-iron ferredoxin, and 4-iron ferredoxin when treated with mineral acid, respectively, are 0,2,4.

Concept:

• Ferredoxins are soluble and iron based proteins containin iron-sulphur clusters.
• These function in electron transfer in photosystem during photosynthesis.
• Three types of ferredoxin proteins have been isolated. These are [2Fe-2S], [3Fe-4S] and [4Fe-4S].
• S in the presentation are labile sulphurs of cysteine which are removed easily on treatment with acid.

Explanation:

Structure of 3-iron ferredoxin is as given below :

Clearly, 3 cysteinyl ligands and 4 sulphur bridges are present in each unit

Conclusion:

Therefore, 3‐iron ferredoxins consist of 4 sulfide bridges and 3 cysteinyl ligands.

CONCEPT:

• Enzymes are biological catalysts that accelerate chemical reactions within living organisms. They are typically proteins, though some RNA molecules can act as enzymes (ribozymes).
• Each enzyme has a specific region called the active site, where the substrate binds and the reaction occurs.
• Enzymes work by lowering the activation energy of a reaction, allowing it to proceed more rapidly. They do this without being consumed or altered in the reaction.
• Many enzymes require metal ions or cofactors to function effectively. These metal ions stabilize reaction intermediates, participate in electron transfer, and activate substrates

EXPLANATION:

Metals in the Active Sites of Specific Enzymes

• Acetylene Hydratase:
  • This enzyme catalyzes the conversion of acetylene (C₂H₂) to acetaldehyde (CH₃CHO) through hydration.
  • The active site of acetylene hydratase contains tungsten (W), a transition metal that facilitates the redox reactions necessary for acetylene hydration. Tungsten is relatively rare in biological systems but is used in a few specialized enzymes.
• Urease:
  • Urease catalyzes the hydrolysis of urea into ammonia (NH₃) and carbon dioxide (CO₂), a reaction important in nitrogen metabolism.
  • The active site of urease contains nickel (Ni). Nickel is essential for binding urea and promoting its decomposition. Urease is one of the most well-known nickel-dependent enzymes.
• Carboxypeptidase:
  • Carboxypeptidase is a protease enzyme that removes the terminal amino acid from the carboxyl (C-terminal) end of peptides and proteins.
  • The active site of carboxypeptidase contains zinc (Zn). Zinc stabilizes the transition state during peptide bond hydrolysis, making it easier for water molecules to attack the peptide bond.
• Sulfite Oxidase:
  • Sulfite oxidase catalyzes the oxidation of sulfite (SO₃^2-) to sulfate (SO₄^2-), a critical reaction in sulfur metabolism.
  • The active site of sulfite oxidase contains molybdenum (Mo). Molybdenum plays a key role in redox reactions, facilitating the transfer of oxygen atoms during the oxidation of sulfite.
• Each of these enzymes relies on a specific metal ion in its active site to catalyze essential biochemical reactions.
• The metal ions act as cofactors, stabilizing reaction intermediates, facilitating electron transfers, and activating substrates for catalysis.
• The metal in each enzyme's active site is chosen based on its chemical properties and ability to interact with the enzyme's substrate or reaction intermediates.

CONCLUSION:

The correct answer is Option 2: W, Ni, Zn and Mo.

Concept:

Hemocyanin:

• From the name, it may suggest that Hemocyanin contains heme and cyanide, but it contains neither.
• The metal atom present in Hemocyanin is Copper and the meaning of Hemocyanin is 'Blue Blood'.
• Hemocyanin is a copper-containing protein that carries oxygen in some invertebrates such as snails, squids, etc.
• The deoxy form of hemocyanin contains Cu in a +1 oxidation state and is colourless.
• The oxy form of hemocyanin contains Cu in +2 form and is bridged to oxygen via an O22--Cu2+ LMCT.

Explanation:

• Hemocyanin molecule has several subunits and binds oxygen cooperatively. 
• The active sites contain two Cu (I) ions 360pm apart for binding of one dioxygen cooperatively.
• Each Cu(I) ion is bound by three histidine residues.
• The dioxygen molecule oxidizes each Cu(I) ion to Cu(II) ion and itself is reduced to the peroxide ion O₂^2-. 
• The peroxide ion bridges two Cu²⁺ ions in theμ−η2:η2−O2−" id="MathJax-Element-8-Frame" role="presentation" style="position: relative;" tabindex="0">μ−η2:η2−O−2μ−η2:η2−O2− $\mu-\eta^2:\eta^2-O_2^-$ mode. The resonance Raman spectroscopy reveals the formulation of Cu-O-O-Cu linkage.
• The 'O-O' stretching frequency in Raman is 745-750 cm^-1, which suggests that the peroxide is not in a free state.
• The UV stretching frequency is about 350-580nm, for the LMCT occurring in oxyhemocyanin imparting it a blue colour.
• The two Cu(II) ions are coupled antiferromagnetically with the μ-O₂^2- ion being involved in a superexchange mechanism.
• As the ions are coupled, there is no magnetic moment observed. The molecule is hence diamagnetic.
• Hemocyanin is colourless in deoxygenated state and blue in an oxygenated state.
• The two states are represented below:

• From the diagram above it is clear that the coordination number of hemocyanin is 5 in oxygenated form.
• Hence, in oxyhemocyanin, the coordination number, mode of oxygen binding, colour and the net magnetic behaviour of copper ions, respectively are five, \(\mu-\eta^2:\eta^2-O_2^-\) blue and diamagnetic.

The correct answer is 1136 and 744 

Concept:-

• Oxygen Binding to Hemoglobin and Hemocyanin: In oxy-hemoglobin, oxygen binds to the iron atom at the center of the heme group. The iron is in the ferrous (Fe2+) state when bound to oxygen, forming a coordinated complex.
• In oxy-hemocyanin, oxygen binds to copper ions in the active site of the protein. Hemocyanin contains copper in the cuprous (Cu+) state.
• Vo-o Resonance Raman Spectroscopy: Resonance Raman spectroscopy is a technique that involves the enhancement of Raman signals through resonance with an electronic transition.
• The Vo-o resonance Raman band specifically involves the stretching vibrations of the oxygen molecule coordinated to the metal ion in the heme or hemocyanin active site.
• Frequency Shifts and Metal-Oxygen Bond Strength: The frequency of the Vo-o resonance Raman band is influenced by the strength of the metal-oxygen bond. A higher frequency indicates a stronger bond.

Explanation:-

In  oxyhemoglobin, the Oxygen molecule binds in bent form (O₂^-) which has a double bond character
oxy-hemocyanin, the Oxygen molecule binds in shared by two hemocyanin group also oxygen is in form (O2-2) which has single bond character.

The Vo-o resonance Raman stretching frequency (cm-¹) of the coordinated dioxygen in oxyhemoglobin and
oxy-hemocyanin appears, respectively nearly at 1136 and 744.

Conclusion:-

So, The vo-o resonance Raman stretching frequency (cm-1) of the coordinated dioxygen in oxy-hemoglobin and oxy-hemocyanin appears, respectively, nearly at 1136 and 744 

Concept:-

Nitrogen fixation:

• Nitrogen fixation is a chemical process that converts atmospheric nitrogen into ammonia, which is absorbed by organisms.
• Nitrogen fixation is essentially converting atmospheric nitrogen into a form that plants can use more easily.

Explanation:-

• The process of biological nitrogen fixation carried out by the nitrogenase enzyme is a complex and energy-intensive reaction. It involves the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃), which is a usable form of nitrogen for living organisms.
• During this process, the enzyme nitrogenase uses ATP (adenosine triphosphate) as an energy source to power the reduction of nitrogen. ATP is a molecule that carries energy in cells and is commonly used in various biochemical reactions.
• The specific number of moles of Mg-ATP required may vary depending on the organism and the specific nitrogenase enzyme involved. It is best to consult scientific literature or research papers that focus on the nitrogen fixation process and the specific enzyme system you are interested in for more detailed and accurate information.

Utilization of 16 ATP in nitrogen fixation:

• Microbes require 16 ATP for the reduction of one mole of nitrogen.
• Nitrogenase enzyme provides 3 hydrogen atoms to each nitrogen atom.
• The 8 electrons come from reduced ferredoxin which is produced in the photosynthesis process.
• Two ATP molecules come from each transferred electron.
• This all sums up in 16 ATP molecules for each nitrogen molecule reduced.
• 

Conclusion:-

So, The number of moles of Mg–ATP needed for the reduction of one mole of nitrogen by the nitrogenase enzyme is 16.